Double folded interdigital delay line



2 Sheets-Sheet 1 Filed Aug. 27, 1965 F/G. i

lNl/EWT'OR TEl-P JAN ATTORNEY April 3 :9 19%? P. JANIS 3,381,159

DOUBLE FOLDED INTERDIGITAL DELAY LINE Filed Aug. 27, 1965 2 Sheets-Sheet 2 FREQUENCY IN MEGACYCLES VOLTAGE REFLECTION FREQUENCY IN MEGACYCLES ATTORNEY United States Patent 3,381,159 DOUBLE FGLDED INTERDIGKTAL DELAY LINE Peter Janis, Auburndale, Mass, assignor to Raytheon Company, Lexington, Mass., a corporation of Delaware Filed Aug. 27, 1965, Ser. No. 483,257 14 Claims. (Cl. SIS-3.5)

The present invention relates generally to traveling wave tubes of the M or 0 type employing electric and magnetic fields together with an interdigital delay line and an adjacent electron beam. In particular, the invention discloses an improved delay line structure which will permit the extension of the operating frequencies of traveling wave tubes to much lower frequencies by increasing the effective electrical length of the radio fre quency delay line path together with additional capacity loading without any alterations of the over-all packaging including external magnetic field producing means.

Traveling wave tubes of the type operating in the backward fundamental space harmonic include a nonresonant periodic slow wave structure or delay line. An elongated sole electrode is spaced from and disposed substantially parallel to said structure and an electron gun is mounted adjacent to one end of the sole electrode to generate an electron beam which traverses the interaction space defined between the slow wave structure and the sole electrode. External permanent magnets or electromagnetic means provide a magnetic field transverse to the electric field and energy is generated or amplified within the tube as a result of the interaction between the electron beam and the high frequency fields of wave energy propagating along the delay line structure. The delay line most commonly employed in such devices comprises opposing interlocking comb-like members defining pluralities of finger elements electrically coupled together in groups to provide a radio frequency wave circuit. The free extremities of alternate finger elements approach but do not contact the common base of the opposing comb member. The capacitance of the circuit is provided by the spacing between the fingers and at the ends of the free extremities while the inductance is provided by the finger elements. Desirably, the overall impedance of the delay line is maintained constant throughout the structure to provide uniform electrical characteristics.

The lateral surfaces defined by the finger elements provide the over-all electrical path to be traversed by the high frequency Waves and there is a direct relationship between the operating frequency of a device employing such delay lines since the low frequency or 1:" mode cutoff is inversely proportional to the capacity between the fingers and the finger length. It has been determined in the art that an over-all length of slightly less than onequarter of the wavelength at the lowest wavelength of the operating frequency band is necessary for an efficient device. At the low frequency end of the microwave fre quency band the electrical length of the delay line finger elements becomes exceedingly long and require packaging requirements with prohibitive costs involved.

In United States Patent No. 2,925,518, issued Feb. 16, 1960, to John M. Osepchuk and assigned to the assignee of the present invention, a new configuration for delay line fingers is disclosed. Specifically, the finger elements comprise a first portion extending perpendicular to a cormnon base or as sometimes referred to, back wall, and attached to said common base. A second portion is defined perpendicular to the first portion and parallel to the common base, while a third portion extends perpendicular to the second portion in a direction extending toward the common base and defining a free space at the extremity thereof. Such delay line elements are referred 3,381,159 Patented Apr. 30, 1968 to as I-shaped members and as a result of this configuration the over-all height of the delay line and magnetic field requirements have been substantially reduced while still providing the necessary electrical characteristics for lower frequency delay line structures. Present day higher power and longer range capabilities have created a need for operative devices in the frequency band of to 450 megacycles. Electrical considerations for delay line structures in such low frequency band devices involve the low frequency cut-off or 1r mode as well as the high starting currents. As for the mechanical design configurations, it is necessary working with present day limitations in the magnet packaging art to increase the overall finger length by a factor in the order of 40 to 60 percent without increasing the magnetic gap requirement.

Briefly, the novel concept of the invention disclosed herein is applicable to slow wave energy propagating structures employed in traveling wave tubes of the type operating in the backward wave fundamental frequency. Such devices may be of the M or 0 type and the invention is equally applicable to both. The free extremity of each delay line finger element is provided with a reentrant portion by means of bifurcating this portion of the delay line element. Additionally, a third plurality of finger elements is provided attached to a common base and extending within the re-entrant portion of each free extremity to result in the double folded slow wave structure. The added surface area defined by the re-entrant wall portions will effectively increase the electrical length of each finger element by the required factor to achieve the lower frequency bands of operation in view of the fact that the radio frequency energy propagated on the line is forced by the opposing third element to transverse this additional surface area. The additional group of elements will provide increased inductance and the spacing between the surfaces of these elements and the re-entrant portion Walls will provide a unique capacity loading means. With the improved structure it is possible where desired to maintain an equal spacing between the finger elements, the free extremity of such elements and the common base as well 'as between the third group of members and the re-entrant wall portions. in other applications the spacings between the third set of members and adjacent Walls may be considerably decreased to capacity load the delay line structure. The incorporation of the disclosed delay line structure in traveling wave tube devices will, moreover, permit a conventional magnetic field strength to be employed while shifting the frequency band to heretofore unattainable values.

For the purposes of this description, the term reentrant shall denote wall structures provided by notching or slotting the free end wall of each interdigital delay line element to define a longer radial radio frequency circuit path within a given tube volume and fixed number of elements thereby increasing the overall electrical length of the delay line structure.

It is an object of the present invention to provide a novel slow wave structure of the interdigital type for traveling wave devices of the backward wave type for operation at lower frequency bands.

A further object of the present invention is the provision in a slow wave structure of the interdigital type of means for varying the frequency cutoff characteristics as well as the starting current requirements.

Other objects, features and advantages will become apparent after consideration of the following detailed description of an illustrative embodiment together with the accompanying drawings in which:

FIG. 1 is a longitudinal cross-sectional view of an illustrative embodiment of the invention;

FIG. 2 is a detailed cross-sectional view along the line 2-2 in FIG. 1;

FIG. 3 is a fragmentary pictorial view of a portion of the improved interdigital delay lines of the invention employed in the device shown in FIGS. 1 and 2;

FIG. 4 is a schematic presentation of a modified version of the invention for linear interdigital delay lines;

FIG. 5 is a schematic presentation of the delay line structure shown in FIG. 4 with additional modifications in accordance with the invention;

FIG. 6 is a schematic presentation of an alternative embodiment of the invention;

FIG. 7 is a graph plotting the dispersion characteristics of the conventional delay line and the embodiment of the invention; and

FIG. 8 is a graph plotting the voltage reflection vs. frequency for the conventional delay line and the illustrative embodiment of the invention.

In the drawings FIGS. 1 and 2 illustrate a backward wave oscillator tube 2 incorporating a slow wave energy propagating structure of the interdigital delay line type. Sole electrode 6 is disposed concentric with the delay line 4 and is normally maintained at a negative potential with respect thereto. An input electrical lead assembly 8, electron gun assembly 10 including a heated cathode 12, magnetic field producing means 16 and output RF coupling means 18 complete the major subassemblies of the over-all embodiment. The circular interdigital delay line 4 comprises several pluralities of interdigital elements secured to a cylindrical .base member 5 which together with the oppositely disposed cover plates 20 and 22 hermetically sealed thereto form the evacuated envelope of the over-all embodiment 2.

The sole electrode 6 comprises a cylindrical portion of an electrically conductive material and includes a web portion 24 bounded by an arcuate portion 25 defining a channel 26 for the purpose of confining the electron beam within the interaction space 28 between the channel Wall surfaces and the delay line 4. One end of a hollow supporting member 30 is inserted within a tubular member 32 which is in turn fixedly secured to the sole electrode I web portion 24. Member 30, in addition to supporting the sole electrode 6, forms a portion of the electrical lead assembly 8 and permits the introduction of external circuit connecting leads to appropriate electrodes within the over-all tube. A slotted section 36 is defined within the sole electrode and the electron gun assembly 10 is positioned therein. This assembly includes a mounting plate 38 to provide means for attachment to the web portion 24 of the sole electrode. The gun assembly includes a cathode, heater, grid and accelerating electrodes, the details of which have not been illustrated. The construction and manner of operation of such an assembly is well known in the art and reference is directed to United States Letters Patent No. 2,914,700, issued Nov. 24, 1959, and assigned to the assignee of the present application, for full clarification.

The input electrical lead assembly 8 comprises sleeve member 40 secured to cover plate 20 together with a dielectric sealing member 42 joined at its outer end to a second electrically conductive sleeve member 44. A terminal glass bead seal 46 supports the electrical leads 34 in spaced relationship and hermetically seals the tube envelope after evacuation.

Output coupling means 18 comprise a coaxial transmission line having an outer conductor 48 and an inner conductor 50 having its inner end secured to an element of the delay line 4 adjacent to the electron gun assembly 10. A collector electrode 52 may be provided for the interception of the electrons after traversal of the arcuate interaction space 28. In the illustrative embodiment a tapered collector is shown as a projection from the base member 5 of the delay line structure 4.

The electric fields established between the delay line structure 4 and the sole electrode 6 may be obtained by means of unidirectional voltage sources biasing the sole electrode negatively with respect to the cathode while the delay line is biased positively relative to both the sole electrode and cathode. The accelerating electrode and grid electrode, as well as cathode and heater, may also be connected by means of leads 34 to suitable voltage biasing sources. Details of the electrical circuitry have been intentionally omitted for purposes of clarity and such knowledge may be obtained by reference to United States Letters Patent No. 2,925,518, issued Feb. 16, 1960, to John M. Osepchuk.

In accordance with the teachings of the invention and referring to FIG. 3 the slow wave propagating structure shown is an interdigital type delay line 4 including a first plurality of parallel periodically spaced finger elements 54 secured at one end to the cylindrical base member 5 with the other end defining a free extremity 55 defining an open-circuit with the adjacent base member wall surface.

The illustrative delay line element is of the J-shaped configuration having a first portion 56 perpendicular to the base member 5 together with a second portion 57 perpendicular thereto and parallel to the base member 5. A third portion 58 extending parallel to the first portion terminates in the free extremity 55. A second plurality of elements 69 of similar construction are disposed adjacent to each of the elements 54 with the free extremity 61 in this instance spaced from the base member 5 adjacent to the upper peripheral wall thereof or the surface opposite to the disposition of the first plurality of elements. This alternating parallel symmetrical array is maintained through-out the delay line structure and is the equivalent of a folded parallel wire transmission line. Each of the delay line elements 54 and 6t) define a re-entrant portion 62 and 63 respectively, by means of bifurcating the free extremity ends. A third plurality of delay line elements or vane members 65 and 66 have one end secured to the base member 5 and the free end extending within and spaced from the .re-entrant wall portions. This array of elements follows the same symmetrical pattern as that of the finger elements and provides a second folded or re entrant line to result in the over-all double folded interdigital delay line structure. The high frequency wave energy propagating along the delay line will therefore follow a path defined by the finger elements and then traverse the re-entrant wall portions to thereby increase the over-all electrical length of each element.

Referring to FIG. 4, a linear array of the embodiment of the .invention is illustrated with opposing comb members 68 and 69 defining pluralities of interdigital finger members 70 and 71 for use in the linear beam or O-type devices. Re-entrant portions 72 and 73 are defined in the free extremity of each finger member and the third plurality of delay line elements 74 and 75 are spaced within each of the re-entrant portions. In FIGS. 3 and 4, embodiments have been illustrated wherein uniform electrical characteristics are maintained throughout the delay line structure by maintaining equal spacings between the first and second plurality of finger elements, between the free extremity ends and the base member wall surfaces and between the re-entrant wall portions and the third plurality of vane members. In addition, the respective height of the third elements and the adjacent finger members are substantially equal and disposed on a common plane.

Referring now to FIGS. 5 and 6, another embodiment of the invention is illustrated for applications where capacity loading is desired of the over-all delay line structure. The distance between the bifurcated ends of the delay line finger elements increases the capacitance of the over-all resonant line circuit. Since the low frequency or 1r mode cutoff is inversely proportional to the capacity between the fingers of the delay line and to the finger length, the increased capacitance will facilitate the attain ment of even lower frequency values. By increasing the width of each of the third plurality of vane members the spacing adjacent the free extremity ends may be controlled to any value desired. Such end capacity loading is illustrated in FIG. 5 for the linear beam type of devices by means of substantially increasing the dimensions of vane members 76 and 77 thereby decreasing the re-entnant wall portion spacings to a value less than the interelement spacings. In this view structure not modified from the view in FIG. 4 has been similarly numbered.

In FIG. 6 a circular array is illustrated wherein finger elements 78 may be provided with a tapered re-entrant portion 79. In this embodiment the vane member positioned within the re-entrant portion will be provided with a similar or pyramidal vane member 80 secured to the cylindrical conductive base member 81. This view illustrates not only the closer spacing technique for capacity loading but also the many possible configurations for the re-entrant wall portions and the conforming vane members. The only requirement is that the spacings be maintained uniformly throughout the additional path provided by the invention without any discontinuities.

The theoretical considerations in the design of delay lines are based on the relationship of the velocity of light 0 to the phase velocity of a wave at a predetermined wavelength v and may be expressed as the rate of c delay It has been determined in the art that this rate is solely a function of the length of the fingers and the pitch of the line. This latter dimension is based on the distance between the axes of two adjacent fingers. In the accompanying graphs a comparison is shown between a delay line of standard construction having a pitch of substantially .340 inch and an improved line incorporating the re-entrant and double folded feature of the invention in the delay line construction while maintaining the same pitch value of .340 inch.

In FIG. 7 the dispersion characteristics are plotted. The theoretical value for is indicated by line 82 and line 84 is the curve for where p equals the pitch of the delay line. The dotted curve 86 indicates the actual measurements with the standard delay line and shows a bandwidth of between 350 to 550 megacycles. Solid curve 88 clearly shows that the frequency coverage with the delay line of the invention has now been substantially extended to a lower limit of 200 megacycles. Tubes may, therefore, be fabricated in the frequency range of 200 to 350 megacycles without any changes in the over-all packaging required simply by incorporating the new delay line structure.

In FIG. 8 dotted curve 90 illustrates the characteristics over a 350-550 megacycle bandwidth for a conventional delay line of voltage reflection versus frequency expressed in percentages. Solid curve 92 illustrates the same measurements with the delay line of the invention. A shift of the lower frequency band limit is indicated and an operative device covering a range from 200 to 400 megacycles is now attainable. Other electrical characteristics such as frequency cutofif and starting current requirements have been satisfied in actual embodiments for lower frequency band operation.

While specific illustrative embodiments of the invention have been described, numerous modifications or alterations may be evident to persons skilled in the art. It is, therefore, intended that the spirit and scope of the invention be interpreted in accordance with the broadest aspects of the definition thereof in the accompanying claims.

What is claimed is:

1. A slow wave energy propagating structure comprising:

a base member:

a first and second plurality of parallel periodically spaced elements arranged in an interdigital array; each of said elements being joined at one end to said base member and having a free termination end spaced from said base member;

said free termination end defining a wall structure providing a re-entrant radial radio frequency energy P a third plurality of periodically spaced elements joined to and extending substantially perpendicularly to said base member;

each of said third plurality of spaced elements extending within and being spaced from the wall surfaces of each of said re-entrant structures.

2. A slow wave energy propagating structure comprising:

a base member;

a first and second plurality of parallel periodically spaced elements arranged in an interdigital array; each of said elements being alternately joined at one end to said base member adjacent to the top and bottom walls and having a free termination end spaced from said base member;

said free termination end defining a wall structure providing a re-entrant radial radio frequency path;

a third plurality of periodically spaced elements joined to and extending substantially perpendicularly to said base member;

each of said third plurality of spaced elements extending within and being spaced from the wall surfaces of each of said re-entrant structures;

the spacings between each of said third plurality of elements and adjacent re-entrant wall surfaces being substantially equal to the interelernent spacing between said first and second pluralities of elements.

3. A slow wave energy propagating structure according to claim 2 wherein:

the spacings between each of said third plurality of elements and adjacent re-entrant wall surfaces are less than the interelement spacing between said first and second pluralities of elements.

4. A slow wave energy propagating structure comprising:

parallel oppositely disposed base members;

a first and second plurality of parallel periodically spaced elements joined to said base members in an interdigital array, each of said elements defining a free termination end spaced from the opposing base member;

wall structure defining a re-entrant radial radio frequency energy path in each of said free termination ends;

a third plurality of periodically spaced elements extending substantially coplanar to said first and second elements and having a free end terminating within each of said re-entrant structures.

5. A slow wave energy propagating structure according to claim 4 wherein the configuration of each of said third plurality of elements conforms to that of the reentrant wall structure.

6. A slow wave energy propagating structure comprising:

a base member;

a first and second plurality of interdigitated periodically spaced substantially J-shaped elements alternately joined to said base member at one end and defining a bifurcated free extremity at the other end;

a third plurality of periodically spaced elements extending substantially perpendicular to said base member and having a free end disposed within each of said bifurcated extremities.

7. A slow wave propagating structure comprising:

a cylindrical base member;

a first and second plurality of interdigitated periodical- 1y spaced substantially J-shaped elements, each of said element ends being alternately short-circuited to said base member adjacent one peripheral wall and open-circuited adjacent the opposing peripheral wall;

wall structure defining a re-entrant portion in each of the open-circuited ends;

a third plurality of periodically spaced elements joined to said base member and having a free end spaced within each of said re-entrant portions.

8. A slow wave propagating structure according to claim 7 wherein each of said third plurality of elements and open-circuited element ends are disposed on a common plane.

9. A slow wave energy propagating structure according to claim 7 in which the height of each of said third plurality of elements and open-circuited elements is substantially equal.

10. A traveling wave crossed-field electron discharge device comprising:

a slow wave energy propagating structure producing in the region adjacent thereto fields of electromagnetic wave energy being propagated therealong;

a source of electrons;

means for directing said electrons in a beam along said region in energy-exchanging relation with said fields of wave energy;

said slow wave structure comprising first and second pluralities of periodically spaced finger elements to define an interdigital array, each of said finger elements defining a bifurcated free extremity;

a plurality of vane members extending within and spaced from the wall surfaces of each of said bifurcated free extremities;

the combined disposition of said finger elements and vane members provided a re-entrant path for electromagnetic wave energy propagated on said slow wave structure.

11. A traveling wave crossed-field electron discharge device according to claim 10 wherein said finger elements and vane members are disposed on a common plane.

12. A traveling Wave crossed-field electron discharge device according to claim 10 wherein the height of each of said finger elements and vane members is substantially equal.

13. A traveling wave crossed-field electron discharge device according to claim 10 wherein the spacing between each of said vane members and adjacent bifurcated wall surfaces is substantially equal to the spacing between said first and second pluralities of finger elements.

14. A traveling wave crossed-field electron discharge device according to claim 10 wherein the spacing between each of said vane members and adjacent bifurcated wall surfaces is less than the spacing between said first and second pluralities of finger elements.

References Cited UNITED STATES PATENTS 2,888,595 5/ 1959 Warnecke et a1. 3153.5

2,925,518 2/ 1960 Osepchuk 31539.73 X

3,068,431 12/ 1962 Watts 33331 3,255,422 6/1966 Feinstein et al. 3l539.3 X

FOREIGN PATENTS 1,194,903 11/1959 France.

HERMAN SAALBACH, Primary Examiner.

S. CHATMON, JR. Assistant Examiner. 

1. A SLOW WAVE ENERGY PROPAGATING STRUCTURE COMPRISING: A BASE MEMBER; A FIRST AND SECOND PLURALITY OF PARALLEL PERIODICALLY SPACED ELEMENTS ARRANGED IN AN INTERDIGITAL ARRAY; EACH OF SAID ELEMENTS BEING JOINED AT ONE END TO SAID BASE MEMBER AND HAVING A FREE TERMINATION END SPACED FROM SAID BASE MEMBER; SAID FREE TERMINATION END DEFINING A WALL STRUCTURE PROVIDING A RE-ENTRANT RADIAL RADIO FREQUENCY ENERGY PATH; A THIRD PLURALITY OF PERIODICALLY SPACED ELEMENTS JOINED TO AND EXTENDING SUBSTANTIALLY PERPENDICULARLY TO SAID BASE MEMBER; EACH OF SAID THIRD PLURALITY OF SPACED ELEMENTS EXTENDING WITHIN AND BEING SPACED FROM THE WALL SURFACES OF EACH OF SAID RE-ENTRANT STRUCTURES.
 10. A TRAVELING WAVE CROSS-FIELD ELECTRON DISCHARGE DEVICE COMPRISING: A SLOW WAVE ENERGY PROPAGATING STRUCTURE PRODUCING IN THE REGION ADJACENT THERETO FIELDS OF ELECTROMAGNETIC WAVE ENERGY BEING PROPAGATED THEREALONG; A SOURCE OF ELECTRONS; MEANS FOR DIRECTING SAID ELECTRONS IN A BEAM ALONG SAID REGION IN ENERGY-EXCHANGING RELATION WITH SAID FIELDS OF WAVE ENERGY; SAID SLOW WAVE STRUCTURE COMPRISING FIRST AND SECOND PLURALITIES OF PERIODICALLY SPACED FINGER ELEMENTS TO DEFINE AN INTERDIGITAL ARRAY, EACH OF SAID FINGER ELEMENTS DEFINING A BIFURCATED FREE EXTREMITY; A PLURALITY OF VANE MEMBERS EXTENDING WITHIN AND SPACED FROM THE WALL SURFACES OF EACH OF SAID BIFURCATED FREE EXTREMITIES; THE COMBINED DISPOSITION OF SAID FINGER ELEMENTS AND VANE MEMBERS PROVIDED A RE-ENTRANT PATH FOR ELECTROMAGNETIC WAVE ENERGY PROPAGATED ON SAID SLOW WAVE STRUCTURE. 